This invention relates to a backscatter thickness measuring instrument and particularly to a direct digital readout automated instrument for the non-destructive measurement of a coating on a substrate which is conditioned or calibrated by computed numerical constants stored in an internal electronic memory.
Backscatter instruments are used to measure thicknesses of coatings by counting particles backscattered from the coatings on a substrate. To determine the thickness of the coating, a user compares the normalized backscatter counts per minute with a plot of coating thickness versus normalized backscattered counts per minute. This plot, which is obtained by fitting a mathematical curve to actual normalized counts of particles backscattered from standards of known thickness, is a non-linear relationship and is unique for each combination of (1) radioactive isotope, (2) coating material, (3) substrate material and (4) the geometry of the measurement situs such as the size of aperture through which the backscattered particles must pass to reach a Geiger counter which counts the backscattered radiation. Hereafter all of these factors which are determinative of the shape of the plot shall be referred to as the "measurement parameters."
It is known in the art that when this non-linear relationship is plotted on semi-logarithmic coordinates (logarithm of thickness versus normalized counts per minute), the plot has one inflection point. For example, see FIG. 1 and 2 of U.S. Pat. No. 3,854,042 which show the general shape of curves of this type. Once the particular semi-logarithmic plot corresponding to a particular combination of measurement parameters is known, it can be defined by two curve constants denoted hereinafter by "A" and "B." The constants "A" and "B" refer to the thickness value at the inflection point on the semi-logarithmic plot and the slope of the curve at the inflection point respectively.
U.S. Pat. No. 3,271,572 disclosed a system that was adapted to provide a direct indication of the coating thickness from the beta particles backscattered from the workpiece to be measured. A meter directly responsive to the beta particles backscattered from the workpiece provided an indication of coating thickness on a readout scale calibrated to conform to the plot of thickness versus normalized beta particle back-scatter counts per minute for the particular measurement parameters. With different measurement parameters the readout scale would be replaced with a readout scale conforming to the new measurement parameters.
U.S. Patent application Ser. No. 631,412 now U.S. Pat. No. 4,079,237, filed Nov. 12, 1975 is an improvement over the system described in U.S. Pat. No. 3,271,572. This improved apparatus provides for a card controlled direct digital readout apparatus for automatically displaying the thickness of coatings. To calibrate this system for different measurement parameters, a card having the calibrating information thereon is read into the system by a card reader. In using this system a card having the calibrating information must be read each time a measurement is made.
A disadvantage of the prior art systems is that the user must calibrate the measuring apparatus each time a measurement is made. With the meter system, the readout scale must be changed for different measurement parameters, and with the card system the appropriate card must be placed in the card reader. In other systems, such as that disclosed in U.S. Pat. No. 3,854,042, calibrating information is entered into the system by dials. User error is introduced, because this calibration information must be determined by the user from tables or otherwise each time a measurement is made.